1. Field of the Invention
The present invention relates to a thermoelectric refrigerating device consisting of composite elements of semiconductors and good conductive material as its heat sink.
2. The Prior Art
At present, all types of high performance domestic refrigerators in the world use compressors (vapor compression systems) working with a refrigerant as heat sinks. Because the unavoidable leakage of the refrigerant, freon, during its manufacture and utilization is badly damaging to the protective ozone layer in the atmosphere, the use of freon has been a cause for great concern in many countries. Replacement of vapor compression refrigerators and thereby the elimination of the use of the refrigerant, freon, has become an important technological problem to be solved in the refrigeration industry.
Until now, thermoelectric refrigeration has not been so developed to be widely applicable in common refrigerators. The major reason for this is that the current P and N elements, which compose a thermocouple, are made only by two single semiconductive materials. Because they are limited by the thermoelectric and processing characters of semiconductive materials, the single element of semiconductive materials cannot be finely composed. Consequently, a reasonable structure conforming to the requirements of practical temperature distribution, cannot be formed of the thermoelectric device consisting of said thermocouples, which are composed of P and N elements. The explanation is as follows:
(1) The heat transfer capacity of the electron flow in semiconductor elements (main component is bismuth telluride), for a given semiconductor material, is such that the heat transfer capacity increases with the cross-section area of P-N couple elements;
(2) The heat transfer speed of the electron flow in P-N couple elements is such that the heat transfer speed decreases with increasing length of the elements;
(3) The temperature difference between the hot and cold end surfaces of P-N couple elements is such that the temperature difference increases in relation to the length of the elements in their heat transfer direction;
(4) The distribution of the heat flow density of a thermoelectric device consists of a multiplicity of dozens or hundreds of P-N couples, such that the distribution of heat flow density in P-N couples depends on the distances between P and N type elements, between P-N couples and between the hot and cold end surfaces of the P and N type elements. In general, the better the conditions for the aforementioned distances are satisfied, the more reasonable the distribution of heat flow density will be; and
(5) The heat rejection rate of the thermal radiator is designed according to the distribution of heat flow density at the hot end of the refrigerating device.
All of the five aforementioned factors relate to one another. When the end area of a thermoelectric element is enlarged to optimize Factor 1 while maintaining the distances between P and N elements and between P-N couples for the multiplicity of dozens or hundreds of P-N couples, the end surface area of the thermoelectric module will be large, and also the distance between the end surfaces of cold and hot will be increased for good insulation; otherwise, the serious heat exchanging will occur inside the thermoelectric module, due to the enlargement of the two ends of the surface area and the temperature difference, which will also be affected. For this reason, the increase of factor 3 must have a big distance interval of insulation and more difference in temperature. But, it is limited by factor 2 if the single semiconductor material will be used to increase the distance interval. Meanwhile, it is also limited by the fragility and processing difficulty of semiconductive material; as a result, factor 4 is only an ideal model and cannot be practiced. In fact, the distance between the hot and cold ends can only be several millimeters in length if the single semiconductive material is used. Thus, it is preferable to use a small distance interval and vacuum insulation or insulation in firmament, and it is extremely unfavorable to use this in common refrigerators.
As a result of the mutual restrictions among Factors 1 to 4, a compromise has to be made. No optimized construction of broad applicability for a thermoelectric module was every found. When direct current flows in a thermoelectric module, the distribution of heat flow density is not reasonable. The temperature difference between cold and hot end surfaces is not large, and there exists considerable heat exchange inside the module. In addition, the air-cooled thermal radiators adopted, at present, are unable to adapt to the distribution of heat flow density at the surface of a thermoelectric module, nor can they meet the required rate of heat rejection. They can only satisfy one of the two aspects, at the expense of the other, consequently aggravating the restrictions of Factors 1 to 4.
The final results are as follows: The practical temperature difference of a thermoelectric module is far below the theoretical maximum value. The cooling efficiency is considered decreased to far below the theoretical value (80 percent). The maximum efficiency achievable on a thermoelectric module, at present, is very low, in comparison to that of vapor compression refrigerators. In addition, the efficiency of a thermoelectric module is dependent upon ambient temperatures. When the ambient temperature is over 25.degree. C., a thermoelectric module will lose almost all its cooling capacity and waste electric energy. If circulating water is applied as coolant for a thermoelectric module, a water supply is required. This not only wastes water, but it also makes it difficult to design and manufacture mobile and portable self-supporting units.
Due to all the aforementioned reasons, state of the art technology in semiconductive thermoelectric refrigeration is still not comparable to that of vapor compression refrigeration and, therefore, has not found broad application in the refrigeration industry.